1,693 research outputs found
Luminosity Profiles of Merger Remnants
Using published luminosity and molecular gas profiles of the late-stage
mergers NGC 3921, NGC 7252 and Arp 220, we examine the expected luminosity
profiles of the evolved merger remnants, especially in light of the massive CO
complexes that are observed in their nuclei. For NGC 3921 and NGC 7252 we
predict that the resulting luminosity profiles will be characterized by an
r^{1/4} law. In view of previous optical work on these systems, it seems likely
that they will evolve into normal ellipticals as regards their optical
properties. Due to a much higher central molecular column density, Arp 220
might not evolve such a ``seamless'' light profile. We conclude that
ultraluminous infrared mergers such as Arp 220 either evolve into ellipticals
with anomalous luminosity profiles, or do not produce many low-mass stars out
of their molecular gas complexes.Comment: Final refereed version. Note new title. 4 pages, 2 encapsulated color
figures, uses emulateapj.sty. Accepted to ApJL. Also available at
http://www.cv.nrao.edu/~jhibbard/Remnants/remnants.htm
The Neutral Hydrogen Distribution in Merging Galaxies: Differences between Stellar and Gaseous Tidal Morphologies
We have mapped the neutral atomic gas (HI) in the three disk-disk merger
systems NGC 520, Arp 220, and Arp 299. These systems differ from the majority
of the mergers mapped in HI, in that their stellar and gaseous tidal features
do not coincide. In particular, they exhibit large stellar tidal features with
little if any accompanying neutral gas and large gas-rich tidal features with
little if any accompanying starlight. On smaller scales, there are striking
anti-correlations where the gaseous and stellar tidal features appear to cross.
We explore several possible causes for these differences, including dust
obscuration, ram pressure stripping, and ionization effects. No single
explanation can account for all of the observed differences. The fact that each
of these systems shows evidence for a starburst driven superwind expanding in
the direction of the most striking anti-correlations leads us to suggest that
the superwind is primarily responsible for the observed differences, either by
sweeping the features clear of gas via ram pressure, or by excavating a clear
sightline towards the starburst and allowing UV photons to ionize regions of
the tails.Comment: 16 pages, 5 figures, uses emulateapj.sty. To appear in the March 2000
issue of AJ. Version with full resolution figures is available via
http://www.cv.nrao.edu/~jhibbard/HIdisp/HIdisp.htm
A new red giant-based distance modulus of 13.3 Mpc to the Antennae galaxies and its consequences
The Antennae galaxies are the closest example of an ongoing major galaxy
merger, and thereby represent a unique laboratory for furthering the
understanding of the formation of exotic objects (e.g., tidal dwarf galaxies,
ultra-luminous X-ray sources, super-stellar clusters, etc). In a previous paper
HST/WFPC2 observations were used to demonstrate that the Antennae system might
be at a distance considerably less than that conventionally assumed in the
literature. Here we report new, much deeper HST/ACS imaging that resolves the
composite stellar populations, and most importantly, reveals a well-defined red
giant branch. The tip of this red giant branch (TRGB) is unambiguously detected
at Io(TRGB)=26.65 +/- 0.09 mag. Adopting the most recent calibration of the
luminosity of the TRGB then yields a distance modulus for the Antennae of
(m-M)o= 30.62 +/- 0.17 corresponding to a distance of 13.3 +/- 1.0 Mpc. This is
consistent with our earlier result, once the different calibrations for the
standard candle are considered. We briefly discuss the implications of this now
well determined shorter distance.Comment: 11 pages, 3 figures, accepted for publication in the Ap
The Revealing Dust: Mid-Infrared Activity in Hickson Compact Group Galaxy Nuclei
We present a sample of 46 galaxy nuclei from 12 nearby (z<4500 km/s) Hickson
Compact Groups (HCGs) with a complete suite of 1-24 micron 2MASS+Spitzer
nuclear photometry. For all objects in the sample, blue emission from stellar
photospheres dominates in the near-IR through the 3.6 micron IRAC band.
Twenty-five of 46 (54%) galaxy nuclei show red, mid-IR continua characteristic
of hot dust powered by ongoing star formation and/or accretion onto a central
black hole. We introduce alpha_{IRAC}, the spectral index of a power-law fit to
the 4.5-8.0 micron IRAC data, and demonstrate that it cleanly separates the
mid-IR active and non-active HCG nuclei. This parameter is more powerful for
identifying low to moderate-luminosity mid-IR activity than other measures
which include data at rest-frame lambda<3.6 micron that may be dominated by
stellar photospheric emission. While the HCG galaxies clearly have a bimodal
distribution in this parameter space, a comparison sample from the Spitzer
Nearby Galaxy Survey (SINGS) matched in J-band total galaxy luminosity is
continuously distributed. A second diagnostic, the fraction of 24 micron
emission in excess of that expected from quiescent galaxies, f_{24D}, reveals
an additional 3 nuclei to be active at 24 micron. Comparing these two mid-IR
diagnostics of nuclear activity to optical spectroscopic identifications from
the literature reveals some discrepancies, and we discuss the challenges of
distinguishing the source of ionizing radiation in these and other lower
luminosity systems. We find a significant correlation between the fraction of
mid-IR active galaxies and the total HI mass in a group, and investigate
possible interpretations of these results in light of galaxy evolution in the
highly interactive system of a compact group environment.Comment: 20 pages, 17 figures (1 color), uses emulateapj. Accepted for
publication by Ap
Star Clusters in the Tidal Tails of Interacting Galaxies: Cluster Populations Across a Variety of Tail Environments
We have searched for compact stellar structures within 17 tidal tails in 13
different interacting galaxies using F606W- and F814W- band images from the
Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST). The
sample of tidal tails includes a diverse population of optical properties,
merging galaxy mass ratios, HI content, and ages. Combining our tail sample
with Knierman et al. (2003), we find evidence of star clusters formed in situ
with Mv < -8.5 and V-I < 2.0 in 10 of 23 tidal tails; we are able to identify
cluster candidates to Mv = -6.5 in the closest tails. Three tails offer clear
examples of "beads on a string" star formation morphology in V-I color maps.
Two tails present both tidal dwarf galaxy (TDG) candidates and cluster
candidates. Statistical diagnostics indicate that clusters in tidal tails may
be drawn from the same power-law luminosity functions (with logarithmic slopes
~ -2 - -2.5) found in quiescent spiral galaxies and the interiors of
interacting systems. We find that the tail regions with the largest number of
observable clusters are relatively young (< 250 Myr old) and bright (V < 24 mag
arcsec^(-2)), probably attributed to the strong bursts of star formation in
interacting systems soon after periapse. Otherwise, we find no statistical
difference between cluster-rich and cluster-poor tails in terms of many
observable characteristics, though this analysis suffers from complex,
unresolved gas dynamics and projection effects.Comment: Accepted for publication in the Astrophysical Journal. 27 pages, 8
figure
Hubble Space Telescope Imaging of the Ultracompact Blue Dwarf Galaxy HS 0822+3542: An Assembling Galaxy in a Local Void?
We present deep U, narrow-V, and I-band images of the ultracompact blue dwarf
galaxy HS 0822+3542, obtained with the Advanced Camera for Surveys / High
Resolution Channel of the Hubble Space Telescope. This object is extremely
metal-poor (12 + log(O/H) = 7.45) and resides in a nearby void. The images
resolve it into two physically separate components that were previously
described as star clusters in a single galaxy. The primary component is only
\~100 pc in maximum extent, and consists of starburst region surrounded by a
ring-like structure of relatively redder stars. The secondary component is ~50
pc in size and lies at a projected distance of ~80 pc away from the primary,
and is also actively star-forming. We estimate masses ~10^7 M(sol) and ~10^6
M(sol) for the two components, based on their luminosities, with an associated
dynamical timescale for the system of a few Myr. This timescale and the
structure of the components suggests that a collision between them triggered
their starbursts. The spectral energy distributions of both components can be
fitted by the combination of recent (few Myr old) starburst and an evolved
(several Gyr old) underlying stellar population, similar to larger blue compact
dwarf galaxies. This indicates that despite its metal deficiency the object is
not forming its first generation of stars. However, the small sizes and masses
of the two components suggests that HS 0822+3542 represents a dwarf galaxy in
the process of assembling from clumps of stars intermediate in size between
globular clusters and objects previously classified as galaxies. Its relatively
high ratio of neutral gas mass to stellar mass (~1) and high specific star
formation rate, log(SFR/M(sol) = -9.2, suggests that it is still converting
much of its gas to stars.Comment: 11 pages, 2 figures, accepted for publication in Astrophysical
Journal Letter
Disruption of Star Clusters in the Interacting Antennae Galaxies
We reexamine the age distribution of star clusters in the Antennae in the
context of N-body+hydrodynamical simulations of these interacting galaxies. All
of the simulations that account for the observed morphology and other
properties of the Antennae have star formation rates that vary relatively
slowly with time, by factors of only 1.3 - 2.5 in the past 10^8 yr. In
contrast, the observed age distribution of the clusters declines approximately
as a power law, dN/dt \propto t^{gamma} with gamma = -1.0, for ages 10^6 yr \la
t \la 10^9 yr. These two facts can only be reconciled if the clusters are
disrupted progressively for at least 10^8 yr and possibly 10^9 yr. When we
combine the simulated formation rates with a power-law model, f_surv \propto
t^{delta}, for the fraction of clusters that survive to each age t, we match
the observed age distribution with exponents in the range -0.9 \la delta \la
-0.6 (with a slightly different delta for each simulation). The similarity
between delta and gamma indicates that dN/dt is shaped mainly by the disruption
of clusters rather than variations in their formation rate. Thus, the situation
in the interacting Antennae resembles that in relatively quiescent galaxies
such as the Milky Way and the Magellanic Clouds.Comment: 9 pages, 5 figures, 1 table, accepted for publication in the
Astrophysical Journal, including revisions after referee repor
Gully Formation at the Haughton Impact Structure (Arctic Canada) Through the Melting of Snow and Ground Ice, with Implications for Gully Formation on Mars
The formation of gullies on Mars has been the topic of active debate and scientific study since their first discovery by Malin and Edgett in 2000. Several mechanisms have been proposed to account for gully formation on Mars, from dry mass movement processes, release of water or brine from subsurface aquifers, and the melting of near-surface ground ice or snowpacks. In their global documentation of martian gullies, report that gullies are confined to ~2783S and ~2872N latitudes and span all longitudes. Gullies on Mars have been documented on impact crater walls and central uplifts, isolated massifs, and on canyon walls, with crater walls being the most common situation. In order to better understand gully formation on Mars, we have been conducting field studies in the Canadian High Arctic over the past several summers, most recently in summer 2018 and 2019 under the auspices of the Canadian Space Agency-funded Icy Mars Analogue Program. It is notable that the majority of previous studies in the Arctic and Antarctica, including our recent work on Devon Island, have focused on gullies formed on slopes generated by regular endogenic geological processes and in regular bedrock. How-ever, as noted above, meteorite impact craters are the most dominant setting for gullies on Mars. Impact craters provide an environment with diverse lithologies including impact-generated and impact-modified rocks and slope angle, and thus greatly variable hill slope processes could occur within a localized area. Here, we investigate the formation of gullies within the Haughton impact structure and compare them to gullies formed in unimpacted target rock in the nearby Thomas Lee Inle
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